Color-forming di- and triarylmethane compounds possessing certain S-containing ring-closing moieties, namely, a thiolactone, dithiolactone or thioether ring-closing moiety are disclosed in International Application No. PCT/US86/02685 published June 18, 1987 as International Publication No. WO87/03541. These dye precursors are rendered colored by contacting with a Lewis acid material capable of opening the S-containing ring moiety. Preferably, the Lewis acid material is a metal ion of a heavy metal with silver ion being particularly preferred.
As disclosed therein, these dye precursors and Lewis acid materials find utility in a wide variety of color-forming applications including the formation of dye images where color formation is brought about in an imagewise fashion by the imagewise application of heat, pressure or other stimulus necessary to effect contact between the dye precursor and Lewis acid material. In a preferred embodiment, thermographic recording materials are provided which employ these dye precursors, particularly the thiolactones such as the thiophthalides in combination with an organic silver salt such as silver behenate. Preferably, these recording materials also include a polymeric binder such as polyvinylbutyral and a heat-fusible organic acidic material such as 3,5-diisopropylsalicylic acid or 4,4'-sulfonyldiphenol.